1. Field of the Invention
The present invention relates to an image processing method and apparatus particularly for color image processing.
2. Related Background Art
In a color image input/output system such as a color scanner and laser color printer, a density signal for each color read from a color original by using a reader (e.g., color scanner) is subjected to gradation processing and thereafter, is supplied to an image output device (e.g., a laser color printer). In most cases, the density signal is corrected by using a gradation conversion table so as to obtain a final output image having a visually proper or favorable quality (final output image is an image directly outputted from a laser color printer, or a print obtained from a dot matrix of a color scanner).
Specifically, in a color print system using a color scanner, a scanner head is manually set at a particular point on a scanner drum to check the final dot percent for each color (e.g., yellow, cyan, magenta). If it is determined that a desired final dot percent cannot be obtained, the values in a gradation conversion table are manually corrected by actuation of an adjusting knob. Alternatively, in a system using a layout scanner, an original is displayed on a graphics display to designate the particular point with a light pen, joy stick or the like and to correct the gradation table in the similar manner as above.
As a method of automatically correcting the gradation table, there is known a method whereby an average value for each color R (red), G (Green), B(Blue), or Y(Yellow), M(magenta), C(Cyan) of image data is obtained to give a bias to the image data (i.e., to perform a parallel displacement of the gradation table). There is also known a method whereby the gradation table is corrected such that the highlight point (with lowest density value or with pixels whose values are smaller than a predetermined number) and shadow point (with highest density value or with pixels whose values are larger than a predetermined value) of image data take the maximum and minimum value (255 and 0 for 8 bit digital data, respectively).
According to the above conventional methods, the gamma conversion is carried out for each image plane, R, G and B (or Y, M and C), by using the gradation table to thereby obtain a visually fine or favorable image quality. These methods therefore do not consider an automatic correction of color hue. In other words, masking is not carried out automatically.